As cellular phones have increased in popularity, manufacturers are including additional functionality, e.g. photography, email, and addressbook. These added features are required to fit into a housing that comfortably fits in a hand. As a result, designers are driven to use the interior space of the housing as efficiently as possible.
One way improve the use of the interior space of a cellular phone housing is to evaluate the efficiency of existing circuitry, e.g. RF power detection. FIG. 1 illustrates a prior art example for detecting RF power. A directional coupler, e.g. a passive structure, is placed between the amplifier and the load. The coupler diverts ˜1% of the forward power to the voltage detector, passing most of the power through to the load. The coupler rejects reflections or reverse power. Thus, the voltage measurement depends on only the incident forward power, a desirable feature. The coupler, however, introduces power loss and is a physically large component in the circuit, having a size that depends upon the operating wavelength.
FIG. 2 illustrates another prior art technique. The voltage detector is directly connected to the amplifier output. This circuit has less loss and smaller size than the coupler circuit of FIG. 1, but the detected voltage is affected by the reflected power from a mismatched load. The power indication will be inaccurate when the load is mismatched.